1.Field of the Invention
The present invention is concerned with an improved ethane conversion process. More particularly, this concept deals with a novel processing scheme to convert light gases such as ethane to aromatics an C.sub.3 + hydrocarbons.
2. Description of the Prior Art
Ethylene is commonly produced by steam pyrolysis or thermal cracking of hydrocarbons, particularly of ethane. Pyrolysis reactor operation conditions and feedstock properties control the composition of the product mixture. High selectivity for the desired product (i.e., ethylene) and minimum coke production are promoted by operating at high temperatures, short residence times, and low hydrocarbon partial pressures. The pyrolysis is usually conducted at pressures close to atmospheric (e.g., from about 20 to 40 p.s.i.g.) and at process temperatures from about 1300.degree. to 1600.degree. F. Many types of pyrolysis reactors are known in the art including fired tubular heaters, pebble-bed heaters, and regenerative furnaces, but fired tubular heaters are the generally preferred type of reactor. Single-pass conversions in these known processes are usually from 55 to 65 percent. When ethane is employed as the feedstock, ethylene yields of less than 50 percent are typical. Accordingly, recycling is employed to maximize the ethylene yield. When processing light hydrocarbon feedstocks such as ethane, ultimate yields of approximately 80 weight percent are possible with recycle cracking if once-through conversion is kept below 60 percent with low pressure operation. Steam is added to the hydrocarbon feed to reduce the hydrocarbon partial pressure; steam-to-hydrocarbon feed ratios are generally 0.1-0.4:1 on a weight basis for light hydrocarbon feedstocks such as ethane and propane.
Following thermal cracking, the effluent from the pyrolysis reactor must be rapidly cooled to a temperature at which no additional reaction occurs. This rapid cooling may be effected by various means, such as by directly admixing the effluent with a cool liquid or by indirect heat exchange or by combinations of these means. Ordinarily, it is desirable to first cool the effluent in transfer line exchangers which generate heat pressure steam and then further cool the exchanged effluent by a direct water quench in a quench tower.
Recovery, separation, and purification of the pyrolysis products are major elements of conventional ethylene manufacturing processes. The system must treat not only a full range of hydrocarbons such as hydrogen, methane, ethylene, ethane, propylene, propane, butylenes-butanes, and C.sub.5 -400.degree. F gasoline, but also minor contaminants such as acid gases, acetylene, propadiene, and hydrocarbon polymers. Broadly, there are three principal separations to be made following the quench and heat recovery system discussed above: (1) gasoline and heavier fractions from the C.sub.4 and lighter hydrocarbons; (2) methane and hydrogen off-gases from the ethylene and heavier hydrocarbons; and (3) ethylene from ethane and the heavier hydrocarbons. These are difficult separations, usually accomplished by low-temperature, high-pressure straight fractionation.
Process flow descriptions and diagrams for typical ethylene manufacturing plants are presented in the 1975 Petrochemical Handbook, Hydrocarbon Processing 54(11): pp. 141-43, November, 1975. A more general discussion of thermal cracking of hydrocarbons to produce ethylene is presented in Encyclopedia of Chemical Technology, ed. by Kirk and Othmer, Vol. 8, 1965, pp. 503-514.
It has also been known for some time that synthetic zeolites may be suitably used to produce high yields of C.sub.3 + hydrocarbons containing a substantial quantity of aromatics from a variety of hydrocarbon feedstocks. For example, U.S. Pat. No. 3,760,024 discloses a process for the preparation of aromatic compounds which involves contacting a feed consisting essentially of C.sub.2 -C.sub.4 paraffins and/or olefins with a crystalline aluminosilicate of the ZSM-5 type at a temperature of 100.degree. to 700.degree. C, a pressure of 0-1000 psig, a WHSV of 0.5-400, and a hydrogen to hydrocarbon ratio of 0-20 and recovering the aromatics produced.